1. Field of the Invention
This invention generally relates to a method and an apparatus for controlling a recording laser power. This invention specifically relates to a method and an apparatus for controlling the recording power of a laser beam applied to a disc-shaped recording medium or an optical disc such as a DVD-R, a DVD-RW, a DVD+RW, or a DVR-Blue.
2. Description of the Related Art
Optical discs contain a CD (compact disc) and a DVD (digital versatile disc). A CD-RW, a DVD-R, a DVD-RW, a DVD+RW, and a DVR-Blue are disc-shaped recording mediums of an organic dye type or a phase change type which each have a continuous recording track and address information intermittently formed along a side of the track at intervals corresponding to a prescribed recording unit.
There is a drive apparatus for optically recording and reproducing an information signal on and from such a disc-shaped recording medium while applying a laser beam thereto. During the recording of the information signal on the disc-shaped recording medium, the drive apparatus modulates the laser beam in accordance with the information signal. The drive apparatus includes an optical pickup for emitting the laser beam toward the disc-shaped recording medium and receiving a laser beam returned therefrom. The quality of the recording of the information signal on the disc-shaped recording medium depends on various factors such as the physical and optical characteristics of the disc-shaped recording medium, the performance of the optical pickup, and the wavelength of the laser beam. Thus, the optimum power of the laser beam for recording the information signal varies from medium to medium, and also varies from pickup to pickup.
Accordingly, it is desirable to determine the optimum value of the recording power of a laser beam when a disc-shaped recording medium is placed in a drive apparatus. Furthermore, it is desirable to control the recording power of the laser beam at the determined optimum value during the actual recording of an information signal on the disc-shaped recording medium. An example of the control of the recording laser beam power is of a feedback type including a step of detecting the actual value of the recording laser beam power, a step of comparing the detected actual power value with the optimum power value, and a step of adjusting the actual power value in response to the result of the comparison.
There are various methods of determining the optimum value of the recording power of a laser beam for a disc-shaped recording medium of an organic dye type or a phase change type.
Some recordable optical discs are designed to be scanned by a laser beam at a constant linear velocity (CLV). Typical examples of such recordable optical discs are a CD-R and a CD-RW. It is known that a recordable optical disc is driven while being scanned by a laser beam at a constant linear velocity higher than the standard value.
Japanese patent application publication number P2001-331940A discloses an optical disc having a program area and a test area extending outward of the program area. In Japanese patent application P2001-331940A, test recording and reproduction are implemented before main-data recording. During the test recording and reproduction, the optical disc remains scanned by a laser beam at a prescribed linear velocity higher than the standard value, and a test signal is recorded plural times on the test area of the optical disc as the recording power of the laser beam is changed in turn among different values on a decreasing stepwise basis. Consequently, there are recorded test signals on the test area of the optical disc which correspond to the different values of the recording power of the laser beam, respectively. The recorded test signals are reproduced from the test area of the optical disc. An optimum recording power value of the laser beam for the prescribed linear velocity of the scanning of the optical disc is decided on the basis of the reproduced test signals. During the main-data recording which follows the test recording and reproduction, the recording power of the laser beam is controlled at the decided optimum value.
There is an optical disc having a data area and a test area extending inward of the data area. In some cases, such an optical disc is subjected to test recording and reproduction while being scanned by a laser beam at a constant linear velocity higher than the standard value. Specifically, during the test recording and reproduction, a test signal is recorded plural times on the test area of the optical disc as the recording power of the laser beam is decreased stepwise. Consequently, there are recorded test signals on the test area of the optical disc which correspond to the different values of the recording power of the laser beam, respectively. The recorded test signals are reproduced from the test area of the optical disc. An optimum recording power value of the laser beam is decided on the basis of the reproduced test signals.
In the case of CLV-based drive of an optical disc, the rotational speed of the optical disc is increased as a portion of the optical disc which is currently scanned by a laser beam radially moves from the outermost position of the optical disc to the innermost position thereof. For example, in the case of CLV-based drive of a DVD-R at a 8-fold linear velocity which means a linear velocity equal to 8 times the standard value, the rotational speed of the optical disc is increased to about 12,000 rpm when the innermost portion of the optical disc is scanned by a laser beam. Spontaneous vibration of an optical disc is greater as the rotational speed thereof is higher. Thus, in the case where a test area of the DVD-R extends inward of a data area thereof, the rotational speed of the DVD-R is relatively high and the spontaneous vibration thereof is relatively great during test recording and reproduction for which the test area of the DVD-R is scanned by a laser beam. The great spontaneous vibration of the DVD-R causes the formation of pits therein to be unstable. The unstable formation of pits means the unstable test recording and reproduction.
In Japanese patent application P2001-331940A, the test area of an optical disc extends outward of the program area thereof. Thus, the rotational speed of the optical disc is relatively low and the spontaneous vibration thereof is relatively small during the test recording and reproduction. Consequently, the test recording and reproduction is prevented from going unstable.
Japanese patent application P2001-331940A also discloses an optical disc having a program area, an inner test area extending inward of the program area, and an outer test area extending outward of the program area. The sizes of the inner and outer test areas of such an optical disc are finite. Therefore, under some recording conditions, one of the inner and outer test areas has been fully used for test recording and reproduction, and can not be used any more. Especially, in the case of high-speed drive of the optical disc, the strategy (the recording laser beam waveform) is complicated and the margin for jitter is small so that a wide area is used for test recording and reproduction. Thus, in this case, it is usual that one of the inner and outer test areas of the optical disc has been fully used for test recording and reproduction and only the other can be used for further test recording and reproduction.
There is an optical disc which can be driven at a constant linear velocity changeable among a 1-fold value (the standard value), a 2-fold value, a 4-fold value, a 6-fold value, and an 8-fold value. An 8-fold-speed optical-disc recorder drives such an optical disc at the 8-fold value. In this case, a required rotational speed of the optical disc is excessively high when an innermost portion of the optical disc is scanned by a laser beam. Accordingly, it is undesirable to implement test recording and reproduction using an inner test area of the optical disc. Thus, in this case, it is desirable to carry out only test recording and reproduction using an outer test area of the optical disc. On the other hand, a 2-fold-speed optical-disc recorder drives the optical disc at the 2-fold value. In this case, a required rotational speed of the optical disc remains acceptable even when an innermost portion of the optical disc is scanned by a laser beam. Thus, in this case, it is acceptable to carry out test recording and reproduction using both the inner and outer test areas of the optical disc. As understood from the above explanation, the speed performance of an optical-disc recorder decides whether or not the inner test area of the optical disc should be used for test recording and reproduction.
Generally, it is difficult to equalize the recording characteristics of the inner and outer test areas of an optical disc such as a DVD-R. In the case where an organic dye film is formed by spin coat during the fabrication of a DVD-R, a recording film in the outer test area is thicker than that in the inner test area. In the case where a resin layer of a polycarbonate substrate for a DVD-R is formed by injection molding from an inner side gate, it is difficult to make uniform the birefringence characteristics throughout a disc surface. Surface vibration and tilt in the outer test area of a DVD-R are greater than those in the inner test area thereof. The outer test area of a DVD-R tends to scratch and be given fingerprints. Therefore, the physical characteristics of the inner test area of a DVD-R differ from those of the outer test area thereof. Thus, it is difficult to obtain exactly equal results of test recording and reproduction using the inner test area of a DVD-R and test recording and reproduction using the outer test area thereof.
A DVD-R or a DVD-RW has a spiral track defined by a continuous groove in a recording disc surface. There are land portions between neighboring groove portions spaced in a radial direction of the disc. The land portions are previously formed with pits called land pre-pits (LPP) representing an address signal indicating the on-disc position of every unit segment of the track. The LPP address signal can be reproduced from the disc while the disc is scanned by a laser beam. The reproduced LPP address signal represents the currently-accessed on-disc position, the information of which can be used for essential purposes. During the recording of a signal on the disc, a laser beam modulated in accordance with the signal to be recorded forms recording marks in the groove of the disc. As the recording power of the laser beam applied to the disc is raised, the size of recording marks formed in the groove increases. When the recording power of the laser beam is raised above a particular value, some of formed recording marks reach land pre-pits (LPP). It is difficult to correctly reproduce the LPP address signal from a disc in such a condition since land pre-pits (LPP) are hardly discriminated from recording marks. During test recording and reproduction, the recording power of the laser beam is set to a maximum level and is then decreased stepwise from the maximum level. Thus, the recording power of the laser beam may exceed the particular value in a former stage of the test recording and reproduction. In this case, there occurs the problem that the LPP address signal can not be correctly reproduced from the disc.